The intent behind using the foam was never based on potential weight savings, although it has turned out to be a side benefit. Initially, I used 1/8" x 3/8" maple glued up in a Warren Truss configuration between the engine mounting beams under the tank, again sandwiched between two layers of 1/32" plywood, and started having fuel foaming problems. This happened on three different models, all using this type crutch. I concluded it was too stiff and transmitting harmonic vibrations straight into the tank. The foam insert solved the problem. Balsa might have worked too, but I never tried it since even contest grade weighs more than the foam. The plywood doesn't significantly contribute to the weight since the maple is relieved down to 1/8" wide under the tank. The composite structure is rigid, light and easy to build flat...and eliminates F1.
Chris